The subject matter disclosed herein relates to improvements in flow controls and valve technology, with particular discussion about methods to detect cycling of a valve assembly onset by use of a tight shut-off mode that automatically closes the valve assembly.
Process lines may include many varieties of flow controls. These process lines typically transfer fluids for use in the chemical industry, refining industry, oil & gas recovery industry, and the like. Examples of the flow controls include pneumatic and electronic valve assemblies (collectively, “valve assemblies”) that regulate a flow of process fluid (e.g., gas and liquid). In conventional implementation, these valve assemblies have a number of components that work together to regulate flow of process fluid into and/or out of the process line. These components include a closure member, a seat, a valve stem, and an actuator. Examples of the closure member may embody a plug, ball, butterfly valve, and/or like implement that can contact the seat to prevent flow. In common construction, the actuator couples with the closure member (via the valve stem). The valve assembly may also incorporate a valve positioner with electrical and/or electro-pneumatic components. During operation, the valve positioner receives control signals from a controller that is part of a process control system (also “distributed control system” or “DCS”). These control signals define operating parameters for the valve assembly, namely, a position for the closure member relative to the seat. In response to the control signal, the valve positioner delivers a pneumatic signal that regulates instrument gas to pressurize the actuator in order to regulate this position.
Problems with the valve assemblies on the process line may disrupt the process and/or prevent the process line from operating in accordance with necessary process parameters. The resulting disruptions can lower yields and reduce quality. In large refineries, chemical plants, and power plants, disruptions can also lead to significant expense from process downtime to troubleshoot and repair the problematic devices. Plant operators therefore have an interest to prevent problems at the device-level before such problems manifest in ways that can hinder operation of the process line.
Designers and manufacturers of valve assemblies and related devices utilize various solutions that are meant to avoid these problems on the valve assemblies. These solutions may instruct operation of the valve assembly in a certain manner (or “mode”) in response to conditions on the device, process line, or process control system in general. One mode is known as tight shut-off. In this mode, the valve positioner attempts to move the valve to a closed position in response to all commanded positions that are below a certain threshold. For example, if the threshold for tight shut-off is 10%, then the valve positioner will attempt to close the valve for commanded positions below 10% and operate as normal for commanded positions above 10%. The tight shut-off mode is useful to prevent operating conditions that arise with the closure member in close proximity to the seat. These operating conditions cause the working fluid to flow at high flow rates and/or velocity. Flow that exhibits these flow parameters can cause wear and/or damage to the valve assembly that can degrade performance and life span of the device.
Unfortunately, use of the tight shut-off mode may inadvertently cause the valve assembly to enter the closed position. This problem may arise because of errors in calibration and/or other manufacturing tolerances that inaccurately characterize the relationship between the measured position and the actual (or “real”) position of the closure member relative to the seat. These errors may indicate that the closure member is closer to the seat than it actually is and, thus, any command signals that instruct a position for the closure member that is very near, but not within, the threshold levels can inadvertently trigger the tight shut-off mode. In certain circumstances, the process control system will continue to deliver command signals that cause the valve assembly to repeatedly cycle into and out of the closed position because the process control system is often not “aware” that the valve assembly is configured to implement the tight shut-off mode.